This project seeks to improve our understanding of the detailed circuitry of the thalamus and its relation to arousal, sleep-wakefulness cycles, and information transfer from subcortical relays to the cerebral cortex. Recent evidence suggests that the reticular nucleus of the thalamus (TRN) participates in the generation of the high-voltage rhythmic activity in the forebrain during slow-wave sleep. The focus of this study is to identify the afferent inputs which might influence the excitability of neurons within the TRN and to determine the details of the relationship of these afferents to the cell bodies and dendrites of TRN neurons. During the past year, the technique of photoconverting intracellularly injected Lucifer yellow dye within neurons in order to make the label electron-dense, and the subsequent processing of tissue for electron microscopic (EM) analysis was perfected, and identified TRN neurons were examined at the EM level with the following goals: (a) to identify the synapses upon different segments of the dendrites of individual TRN neurons, and (b) to determine whether or not some of the finer beaded 'dendrites' of these neurons might give rise to dendrodendritic synapses which could serve to coordinate the activity of neighboring TRN neurons. when identified TRN neurons are examined in the EM, both asymmetric and symmetric synapses are observed upon all dendrites and the density of the synapses is relatively constant along the length of the dendrites. Occasional dendritic branch points, however, appear to be densely innervated by asymmetric synapses whose pre-synaptic element may correspond to D-type terminals. Synaptic contacts were observed upon the swollen, as well as the constricted segments of the beaded dendrites, and the fine beaded dendrites were not found to give rise to pre-synaptic profiles.